Water-soluble vitamin deficiency and toxicity Quiz: Ace Your Exams

An 85-year-old man is brought in by his niece to the clinic after finding him confused outside his house. He denies the incident and says that he was “just walking his dog around the neighborhood” after having a meal with his friends. After the consultation, his niece states that “He doesn't own a dog, he lives alone, and most of his meals consist of reheating canned foods.” Past medical history includes hypertension, hypercholesterolemia and type 2 diabetes. Medications include hydrochlorothiazide, atorvastatin and metformin.  Family history is noncontributory. He has been drinking 5-6 beers daily for the past 30 years. Temperature is 37.0°C (98.6°F), pulse is 92/min, respirations are 17/min, and blood pressure is 135/85 mmHg. He is oriented to place and person but not time. Immediate and delayed memory recall is poor. Pupils are equal and reactive, and horizontal nystagmus is elicited on lateral gaze. Fundoscopy is normal. There is no nuchal rigidity on examination. Motor strength is 5/5 in the upper extremities and 2/4 in bilateral lower extremities. Deep tendon reflexes of the knee and ankle are 1+ bilaterally. MRI reveals necrosis in the mamillary bodies and medial thalamus. Which of the following is the most likely diagnosis?